Standdown AF2  august 2002

Reason for standdown:

During last run timeouts on the theta mechanism were causing problems when starting the instrument up. Faultreports: 15006/15016, during the run timeout period was made longer.

Place:

Test Focal station. 

Work done:

It was decided to do a complete stripdown of the gripper unit. This has never been serviced since it was put into operation in 97. This way a complete inspection and necessary repairs could be caried out. Further it was an oportunity for me personally to  learn more about the mechanisms and the way everything is set up.

The gripper unit was taken off the X-Y slide to be able to work on it on the bench.

Gripper was separated to get acces to all mechanisms.

On disassembly I found out the following:

*The O-rings on the piston were dried out

*Some hard grease was found, this could explain the phenomena we once saw that the gripper jaws don’t open properly (the vent could be blocked by the hard grease).

*A pully on the theta-axis was slightly bent.

*The limitswitch to prevent the Z axis to run into the mech hardstop was broken.

Actions:

*All O-rings were changed and lubricated with vacuum grease

*Surfaces on the piston and cilinder were polished to make sure that the O-rings move smoothly.

*Z limitswitch was changed

*Pully was changed

*The whole unit was cleaned and reassembled.

After this stripdown a series of calibration was needed to make sure that all the mechanisms were zerosetting correctly.

Theta-axis.

First off all was cheched that it moved freely now. Therefor the backlash on the worm gear was adjusted. Belt tension was set up to optimum. Mechanism was checked using engineering terminal.

We resetted the zero position of theta. You do a zeroset and then use the ruler to measure and see if the gripper jaws are orthogonal to the X and Y axis. We had to do a small adjustment of 49 encoder counts, this was corrected in the software.

Tv viewing system.

This system is fitted to the gripper and looks a the fibre end, for picking and placing is is very important that the offset between centre of theta rotation and the tv centre is set up correcly.

A fibre was clamped in the gripper, slightly offset. Then theta was moved through 360 degrees. On the tv screen  the imaged fibre  should describe a circle  with its centrepoint that has  to coincide with the true tv centre. Camera mount was adjusted to achieve this.

Next thing was to check that the camera is orthogonal to the X and Y axis. This is done viewing a fibre and then  move the robot in one axis X or Y and look on the tv screen if the fibre moves either horizontal or vertical.

Z-axis

The limitswitch on Z was replaced. The zerosetswitch had to be removed aswell. Therefore we had to recalibrate this mechanism. This was done with the fibre module on.

The zerosetswitch was adjusted to make sure that there is enough travel on the mechanism to reach the outher limit. We measured the distance from the gripper frame to the gripper jaws with a micrometer. The limitswitch was setup 600 microns further back than the zeroswitch.  Then there’s still another 600 microns before it runs into the hardlimit.

Next thing was to determine the pickup position. We moved the gripper towards the fieldplate above fibre 121. Then we made sure that 75 % of the sharksfin was covered by the jaws. The value was read and changed in software. We made sure that the jaws were aligned aswell. Some more test picking and placing fibres were done.

We checked aswell that the fibre is lifted high enough to clear any other fibres. We’ve seen in the past that fibres catch on each other if they’re not lifted enough.

During testing we’ve found out that the belt tension was set to tight and occasionally the mechanism stalled.

Other things discovered  while testing:

*Bad connector on power amplifier crate: refitted with different powercable.

*Bad connector on power supply in instrument: reseated connector

*Voltage on powersupply to linedriver dropped below 5V: recalibrated potentiometer.

After this we did several setups, small_circle/small_park at different instrument orientations.

Work done during run september 2002

First off all to mencion that the two !! S/D nights were wiped out by bad wheather. Saturday was the first science night and the wheather was better.

I’ll give a summary about the faults that occurred and the actions taken.

Saturday 1-9

Fault 15294; drama task failed. This is a known thing apart from this I discovered from the log that it failed twice earlier on a setup due to a failure on a centroid while putting a fibre down. The next morning I tried to do the setup that failed and was fine.

Sunday 2-9

Fault 15297; robot failure.

According to the log while doing setup a2589_bright.cfg;1 it failed to put down fibre 125. So there;s some suspicion on the Z movement. When it failed to put a fibre down it will retract in Z and sit there with the fibre in the jaws. It won't allow you to do anything until you release the fibre. Later on the VME was rebooted what takes the airpressure off, so the fibre will be released. The gripper status was open now so should be ok to do a wake, still would not work. This morning we tried to wake from engineering terminal wouldn't work. Took off module checked all the belts and that motors were servoing. Put module back on an woke from ICL, no problems. We then investigated the cabling if that was causing the problem, ok. Spent some time on the Z mechanism, readjusted the belt tension although it seemed allright. Did a soak test on waking and sleeping and moving the Z mechanism in and out, no failures. Then did the same field setup that failed previous night, no problem. Did an allcircle and setup of the same field again now with the telescope at zenith, no problem

The only thing that we can think off that could have caused this is the Z limit switch. Altough it seems hard to believe that it was operated if you bear in mind that from 0 (encodercounts) it has a 600 micron clearance and the pickup position is 400. That means there 400 * 1,36 = 544 microns plus 600 microns clearance??

Monday 3-9

No faults

Tuesday 4-9

Fault 15299: failure to ftp files: due to software bug O/S9.

Fault 15300:

We tried the same setup as last night when it failed (a376_bright.cfg;1). We managed to reproduce the fault this time on fibre 14. It failed to centroid. We believe this is because it didn't pick up the fibre, the robot then moves to the putdownposition and the tv camera will look at it but won't see it and come up with teh error "centroid failure". We took off the module, investigated if the airpressure could be causing the problem, but with the current pressure gripper operation was ok. Current pressure is 6 bar. From 4 bar and lower the gripper starts to fail. Checked the cabling on the solenoid that operates the gripper, ok. RAB reduced the Z speed and acceleration and this seems to make the mechanism more smooth. After this run we might have to think abouty replacing the leadscrew on this mechanism, it sounded a bit rough with the old (faster) speed. We did another setup (a2169_faint_recent.cfg;1), this time it failed on fibre 148, pickup error, that indicates that it failed to move down in Z. This could be caused by the fact that it moves down and hits the shoulder on the button, that will cause the Z drive to retract to 400 and it will stay there. We then changed the Z value for pickup. It was at 5500 and we changed it to 5375 which means that there's 170 microns more clearance between the gripper jaws and the button shoulder. Later on we saw that it failed to view a fibre before pickup when it was actually visible on the tv monitor. We checked all the cabling on the VME rack for any loose connections. We changed one of the DMC cards that had an error light on it. Cooling on the VME was improoved. Finally we did a last setup (a2169_faint_recent) and that completed ok. Maybe a matter of overheating of the VME?

Wednesday 5-9

No more faults. Occasionally D task lost. The intervals of this failing got larger. This probably due to the fact that KMD properly terminated the loose end on the network cable at UES.

Thursday 6-9

Ok

Friday 7-9

Ok

Saturday 8-9

Ok

Sunday 9-9

Ok

Conclusion.

It still seems hard to understand what happened on Sunday night when we lost 6 hours. The only thing we can think of is the Z limit switch. Again as I explained above it seems weird, but maybe with the higher speed that we had before it over travelled?  Maybe we have to talk about getting rid of this switch?

I think the other problems that occurred were very random and therefore is difficult to try and find a relation. It was interesting to see that as soon as we changed the cooling on the VME that we didn’t have any more problems. The room where the electronics sit, is v ery full now with all the racks of Wyffos, Naomi, Integral and the Ingrid compressor. The latter is causing a big heatload on all of the electronics. This situation with the coming of Grace will be of the past.

Further we still had the D-task dropping of the network from time to time, what doesn’t make things easier to work. In the end it all contributes to a general feeling of frustration. The same thing for all these error messages you get on  the ICL. We, staff know not to pay to much attention to them, but an observer might get worried and it certainly doesn’t help to make them feel confident about the instrument.

Finally it’s a real shame that we lost the two S/D nights to bad wheather, because I think we probably would have caught some problems in an earlier stage. Especially since Sunday was their first night on their own and was when they really were doing a lot of setups.

Michiel van  der Hoeven            AF2 instrument manager

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